Go to a different LFS in my opinion. We all have different oppinions about how to run our reefs but advising against them seems downright silly. He perhaps doesnt understand how they function and thier purpose. Many people do run nice tanks without them, but generally they are considered a given on most systems nowdays. Is he old school from 20 years back in the saltwater world by chance? Allot has changed.
Exception- if its a small tank then I wouldnt bother either. It really gets a chance to show off its abilities in larger tnaks where doing waterchanges gets expensive and time consuming in a hurry. I would personally put the over under for opinion on skimmers at the 75gallon range. Me myself? I would run one on anyting over a 30g but thats totally just me. Smaller tanks you can just do waterchanges and avoid toxins just as easily.